Compositing Two-Dimensional Materials with TiO 2 for Photocatalysis

Energy shortage and environmental pollution problems boost in recent years. Photocatalytic technology is one of the most effective ways to produce clean energy—hydrogen and degrade pollutants under moderate conditions and thus attracts considerable attentions. TiO2 is considered one of the best photocatalysts because of its well-behaved photo-corrosion resistance and catalytic activity. However, the traditional TiO2 photocatalyst suffers from limitations of ineffective use of sunlight and rapid carrier recombination rate, which severely suppress its applications in photocatalysis. Surface modification and hybridization of TiO2 has been developed as an effective method to improve its photocatalysis activity. Due to superior physical and chemical properties such as high surface area, suitable bandgap, structural stability and high charge mobility, two-dimensional (2D) material is an ideal modifier composited with TiO2 to achieve enhanced photocatalysis process. In this review, we summarized the preparation methods of 2D material/TiO2 hybrid and drilled down into the role of 2D materials in photocatalysis activities.


Introduction
With the massive consumption of fossil energy and serious environmental pollution problems, there is an urgent need for clean energy and more efficient ways to decompose pollutants.Photocatalysis is an advanced technology that uses photon energy to convert chemical reactions occurring under harsh conditions into reactions under mild conditions by appropriate photocatalyst, and thus emerged as recognizable fields such as hydrogen generation [1][2][3][4], sewage treatment [5][6][7], harmful gas removal [8,9], organic pollutant degradation [10][11][12][13] and carbon dioxide reduction [14][15][16].
Since the first report that TiO 2 electrode was applied for hydrogen production by Fujishima and Honda in 1972 [17], TiO 2 has attracted numerous attention in photocatalysis as a typical n-type semiconductor [18][19][20][21].Being non-toxic, inexpensive, highly stable [22][23][24], TiO 2 is widely investigated in photocatalytic fields.Hoffman proposed the following general mechanism (Table 1) for heterogeneous photocatalysis on TiO 2 [25].Where >TiOH represents the primary hydrated surface functionality of TiO 2 , e cb − is a conduction band (CB) electron, etr − is a trapped conduction band electron, h VB + is a valence band (VB) hole, Red is an electron donor, Ox is an electron acceptor, {>Ti IV OH}• + is the surface-trapped VB hole (i.e., surface-bound hydroxyl radical), and {>Ti III OH} is the surface-trapped CB electron.Upon light irradiation, electrons transfer from VB to CB of TiO 2 , while both electrons and holes can be trapped by primary hydrated surface functionality of TiO 2 , achieving the separation of photo induced electrons and holes.At the same time, the recombination between electrons and holes exits, which competes with charge-carrier trapping process.The competition has thus a negative effect on later interfacial charge transfer.Deliberating on TiO 2 photocatalysis process, some drawbacks exit as following: (1) The wide bandgap of TiO 2 (3.2 eV) means that photons with adequate energy can only excite electrons in the VB to the CB of TiO 2 , which limits its effective use of sunlight (UV region, λ ≤ 387 nm); (2) The recombination of excited electrons and holes is inevitable while time for carrier recombination is much shorter than that for charge transfer.Therefore, the effective function of photoexcitation is suppressed greatly.
Considering the above two factors, the improvement of the photocatalytic efficiency of TiO 2 can be obtained through two aspects: the improvement of solar light utilization efficiency and the suppression of recombination of electron and hole pairs.In this text, surface modification and hybridization of TiO 2 such as noble metal loading [26][27][28][29] and semiconductor heterojunction [30][31][32] are effective methods to enhance the photocatalytic performance.The Schottky barrier formed at the interface between the noble metal material and TiO 2 can effectively promote the separation of photogenerated carriers.Similarly, the heterojunction structure can form a matching energy level at the semiconductor interface to suppress the recombination of photogenerated carriers.However, the opportunities of improvements in photocatalysis performances offered by these attempts are narrow, and thus limited their commercial and efficient application.In the past decade, two-dimensional (2D) materials have attracted more and more attention because of the flexible preparation methods, low price and superior physical and chemical properties.In particular, their high surface area, suitable bandgap, structural stability and high charge mobility [33][34][35][36] endow these 2D materials with remarkable performances for applications in photocatalysis [37][38][39][40][41].When combined with TiO 2 , not only the utilization of sunlight is improved, but also the matching between energy levels is formed to inhibit the recombination, and the large specific surface area provides support and active sites for the reaction.In this review, we summarize the recent advances of 2D material-TiO 2 composites, including synthesis methods, properties, and catalytic behaviors.Furthermore, the photocatalytic mechanism is deliberated in detail to elaborate the role of 2D materials in the photocatalytic processes.

2D-Material Modified TiO 2
Based on the mobile dimension of electronics, it can be divided into zero-dimensional (0D) materials, one-dimensional (1D) materials, two-dimensional (2D) material and three-dimensional (3D) materials [36], while 2D materials represent an emerging class of materials that possess sheet-like structures with the thickness of only single or a few atom layers [42].Compared with the bulk structures, the ultrathin 2D structure exhibits superior properties such as modification of energy level and larger adjustable surface area.The excellent properties of 2D materials make them widely used in many aspects [43][44][45].When composited with TiO 2 , the synergistic effect of the two can significantly improve the photocatalytic activity and thus 2D materials is ideal for TiO 2 photocatalysis.

Graphene Modified TiO 2
Since the first isolation by Geim and Novoselov in 2004, graphene has attracted significant attention [46][47][48][49].Graphene is a 2D honeycomb construction consisting of carbon atoms.The thickness of graphene is only 0.335 nm, which is the thickness of a carbon atom layer.In the sp 2 hybrid distribution form, each carbon atom contributes an unbonded π electron, which can delocalize freely throughout the carbon atom 'net' to form an extended π bond.This construction endows graphene excellent properties such as high charge mobility (200,000 cm 2 V −1 s −1 ), high thermal conductivity (5000 W m −1 K −1 ), and large surface area [35], which is ideal for applications in sensors [50], energy conversion and storage [37], polymer composites [51], drug delivery systems [52], and environmental science [53].When composited with TiO 2 , graphene can accept photoinduced electrons from TiO 2 and thus greatly enhances the efficiency of carriers' separation [54][55][56][57][58].

The Synthesis of Graphene/TiO 2 Composites
Graphite oxide and graphene oxide (GO) intermediates are widely used in the process of combining graphene with other materials [59].The most widely used technique is chemical reduction of GO as shown in Figure 1, which is usually conducted by Hummers' method [60].Graphite is added to a strongly oxidizing solution such as HNO 3 , KMnO 4 , and H 2 SO 4 to prepare graphite oxide and the oxygen-containing groups are introduced into the surface or edge of the graphite during the process.The sheets of graphite oxide were exfoliated to obtain GO.The presence of oxygen-containing groups allows GO to provide more surface modification active sites and larger specific surface areas for synthetic graphene-based composites.GO can be converted to reduced graphene oxide (RGO) by chemical reduction to remove these oxygen-containing group.During this process, the number of oxygen-containing groups on the GO decreases drastically, and the conjugated structure of the graphene base will be effectively restored.The presence of oxygen functionalities in GO allows interactions with the cations and provides reactive sites for the nucleation and growth of nanoparticles, which results in the rapid growth of various graphene-based composites.The preparation methods for graphene/TiO 2 composites are divided into ex-situ hybridization and in-situ growth, the difference between which is the process of TiO 2 formation.

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Ex-situ hybridization.The common procedure for ex-situ hybridization is to mix GO and modified TiO 2 with physical process such as ultrasound sonication and heat treatments.Rahmatollah et al. [62] reported a facile one-step solvothermal method to synthesize the TiO 2 -graphene composite sheets by dissolving different mass ratios of GO and TiO 2 nanoparticles in anhydrous ethanol solution.
Ultrasound irradiation was used to disperse the GO.Finally, a six-fold enhancement was observed in the photocurrent response compared to the improved photoelectrochemical performance (3%) with the pure TiO 2 .Florina et al. [63] prepared graphene/TiO 2 -Ag based composites as electrode materials.Similarly, GO suspensions were mixed with prepared TiO 2 -Ag nanoparticles in NaOH solution.The suspensions were sonicated, dried and subjected to thermal treatment.However, the control of modification between the TiO 2 and graphene may lead to a decreased interaction between these two parts [64].

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In-situ growth.The in-situ growth method is widely used to prepare graphene-based composite materials, and the method can effectively avoid clustering of nanoparticles on the surface of graphene.According to different preparation process, it might be divided into reduction method, electrochemical deposition method, hydrothermal method and sol-gel method.

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Reduction method.Usually, in a reduction method, GO and TiO 2 metal salts are mixed as precursors.By controlling the hydrolysis of the precursor, TiO 2 crystal nucleus grows on GO, while GO is reduced to obtain graphene-based TiO 2 composite materials [65].In addition to the chemical reduction method, other commonly used reduction methods are photocatalytic reduction [66] and microwave-assisted chemical reduction [67].

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Electrochemical deposition method.In an electrochemical deposition method, graphene or reduced graphene is used as a working electrode in a dielectric solution containing a metal precursor or its compound [68].

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Hydrothermal/solvothermal method.A hydrothermal/solvothermal method is commonly used for preparing inorganic nanomaterials.It is generally carried out in a dispersion of GO.Under high temperature and high pressure, GO and titanium salt precursor are reduced simultaneously [69,70].

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Sol-gel method.The sol-gel method takes titanium alkoxide or titanium chloride as precursors, and it can be uniformly bonded with oxygen group on graphene, polycondensed to form a gel.Then TiO 2 nanoparticles are formed through calcining [71,72].The sol-gel method can obtain loaded nanoparticles with higher uniformity of dispersion.
2.1.1.The Synthesis of Graphene/TiO2 Composites Graphite oxide and graphene oxide (GO) intermediates are widely used in the process of combining graphene with other materials [59].The most widely used technique is chemical reduction of GO as shown in Figure 1, which is usually conducted by Hummers' method [60].Graphite is added to a strongly oxidizing solution such as HNO3, KMnO4, and H2SO4 to prepare graphite oxide and the oxygen-containing groups are introduced into the surface or edge of the graphite during the process.The sheets of graphite oxide were exfoliated to obtain GO.The presence of oxygen-containing groups allows GO to provide more surface modification active sites and larger specific surface areas for synthetic graphene-based composites.GO can be converted to reduced graphene oxide (RGO) by chemical reduction to remove these oxygen-containing group.During this process, the number of oxygen-containing groups on the GO decreases drastically, and the conjugated structure of the graphene base will be effectively restored.The presence of oxygen functionalities in GO allows interactions with the cations and provides reactive sites for the nucleation and growth of nanoparticles, which results in the rapid growth of various graphene-based composites.The preparation methods for graphene/TiO2 composites are divided into ex-situ hybridization and in-situ growth, the difference between which is the process of TiO2 formation.

The Role of Graphene in TiO 2 Photocatalysis
Due to the large bandgap, the photocatalysis process of pure TiO 2 can only be activated under UV light.Thus, the hybridization of graphene and TiO 2 is essential to ensure a broad light stimulation process.In graphene/TiO 2 system, electrons flow from TiO 2 to graphene through interface because of the higher Fermi level of TiO 2 .Then graphene gains excess negative charges while TiO 2 has positive charges, leading to a space charge layer at the interface which is regarded as Schottky junction.The Schottky junction can serve as an electron trap to efficiently capture the photoinduced electrons [73] and thus enhance the photocatalysis activity.Meanwhile, the Schottky barrier also acts as the main obstruction for the electron transport from the graphene to TiO 2 .Under visible light, electrons on Fermi level of graphene are irradiated and the Schottky barrier has to overcome to ensure the injection of electrons to conduct band of TiO 2 .In the UV light irradiation process, graphene plays a role as electron acceptor and thus promotes the separation of electron-hole pairs [54] (Figure 2).Different interface interactions have been extensively studied [55,56].Compared with 0D-2D Degussa P25 (TiO 2 )/graphene and 1D-2D TiO 2 nanotube/graphene composites, the 2D-2D TiO 2 nanosheet/graphene hybrid demonstrates higher photocatalytic activity toward the degradation of rhodamine B and 2,4-dichlorophenol under the UV irradiation [56].The intimate and uniform contact between the two sheets-like nanomaterials allowed for the rapid injection of photogenerated electrons from the excited TiO 2 into graphene across the 2D-2D interface while achieving effective electron-hole pair separation and promoted radical's generation.In another example of RGO-TiO 2 hybrid, by having a narrower bandgap, the photo-response range of RGO-TiO 2 nanocomposites clearly extended from UV (~390 nm) to visible light (~480 nm), which offered a better utilization of visible light [55].Raman spectra and other characterization revealed that the narrow bandgap was attributed to the Ti-O-C bond between the two components, and thus caught an intimate interaction between TiO 2 nanoparticles and RGO sheets.What's more, the up-conversion photoluminescence (UCPL) effect of RGO assists the light absorption, and enabled the efficient utilization of both UV light and visible light (Figure 3).It is worth to note that the surface area of RGO-TiO 2 was smaller than that of pure TiO 2 (P25), which revealed that the enhanced photocatalytic activity of RGO-TiO 2 was relevant to the improved conductivity and bandgap structure other than their surface area.RGO nanosheet can play a role in both charge transfer and active sites after doping with heteroatoms.TiO 2 /nitrogen (N) doped reduced graphene oxide (TiO 2 /NRGO) nanocomposites was applied to photoreduction of CO 2 with H 2 O vapor in the gas-phase under the irradiation of a Xe lamp (the wavelength range of 250-400 nm) [57].Compared with TiO 2 , TiO 2 /NRGO composites exhibited a narrower bandgap due to chemical bonding between TiO 2 and the specific sites of N-doped graphene.In the photoreduction of carbon dioxide, the function of nitrogen atoms varied in different chemical environments.The pyridinic-N and pyrrolic-N worked as active sites for CO 2 capture and activation while quaternary-N worked as an electron-mobility activation region for the effective transfer of photogenerated electrons from the CB of the TiO 2 [57] (Figure 4).The results reveal that the doped atoms can act as basic sites for anchoring target molecular, adjusting the electronic properties and local surface reactivity of graphene.
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 25 clearly extended from UV (~390 nm) to visible light (~480 nm), which offered a better utilization of visible light [55].Raman spectra and other characterization revealed that the narrow bandgap was attributed to the Ti-O-C bond between the two components, and thus caught an intimate interaction between TiO2 nanoparticles and RGO sheets.What's more, the up-conversion photoluminescence (UCPL) effect of RGO assists the light absorption, and enabled the efficient utilization of both UV light and visible light (Figure 3).It is worth to note that the surface area of RGO-TiO2 was smaller than that of pure TiO2 (P25), which revealed that the enhanced photocatalytic activity of RGO-TiO2 was relevant to the improved conductivity and bandgap structure other than their surface area.RGO nanosheet can play a role in both charge transfer and active sites after doping with heteroatoms.TiO2/nitrogen (N) doped reduced graphene oxide (TiO2/NRGO) nanocomposites was applied to photoreduction of CO2 with H2O vapor in the gas-phase under the irradiation of a Xe lamp (the wavelength range of 250-400 nm) [57].Compared with TiO2, TiO2/NRGO composites exhibited a narrower bandgap due to chemical bonding between TiO2 and the specific sites of N-doped graphene.
In the photoreduction of carbon dioxide, the function of nitrogen atoms varied in different chemical environments.The pyridinic-N and pyrrolic-N worked as active sites for CO2 capture and activation while quaternary-N worked as an electron-mobility activation region for the effective transfer of photogenerated electrons from the CB of the TiO2 [57] (Figure 4).The results reveal that the doped atoms can act as basic sites for anchoring target molecular, adjusting the electronic properties and local surface reactivity of graphene.clearly extended from UV (~390 nm) to visible light (~480 nm), which offered a better utilization of visible light [55].Raman spectra and other characterization revealed that the narrow bandgap was attributed to the Ti-O-C bond between the two components, and thus caught an intimate interaction between TiO2 nanoparticles and RGO sheets.What's more, the up-conversion photoluminescence (UCPL) effect of RGO assists the light absorption, and enabled the efficient utilization of both UV light and visible light (Figure 3).It is worth to note that the surface area of RGO-TiO2 was smaller than that of pure TiO2 (P25), which revealed that the enhanced photocatalytic activity of RGO-TiO2 was relevant to the improved conductivity and bandgap structure other than their surface area.RGO nanosheet can play a role in both charge transfer and active sites after doping with heteroatoms.TiO2/nitrogen (N) doped reduced graphene oxide (TiO2/NRGO) nanocomposites was applied to photoreduction of CO2 with H2O vapor in the gas-phase under the irradiation of a Xe lamp (the wavelength range of 250-400 nm) [57].Compared with TiO2, TiO2/NRGO composites exhibited a narrower bandgap due to chemical bonding between TiO2 and the specific sites of N-doped graphene.
In the photoreduction of carbon dioxide, the function of nitrogen atoms varied in different chemical environments.The pyridinic-N and pyrrolic-N worked as active sites for CO2 capture and activation while quaternary-N worked as an electron-mobility activation region for the effective transfer of photogenerated electrons from the CB of the TiO2 [57] (Figure 4).The results reveal that the doped atoms can act as basic sites for anchoring target molecular, adjusting the electronic properties and local surface reactivity of graphene.Except for dimension factor and bonding interaction between graphene and TiO2, a linkage is introduced to graphene/TiO2 system to achieve better interfacial contact as well.A N-doping Graphene-TiO2 composite nano-capsule for gaseous HCHO degradation was reported [58].It indicated that wrapping with dopamine on the surface of TiO2 enhanced interfacial contact between TiO2 and melamine-doped graphene (MG) sheets, thus promoting the separation and mobility of photoinduced electrons and holes in TiO2@MG-D.The dopamine acted as bridge between TiO2 and MG, creating numerous migration channels for charges and restraining the recombination of electrons and holes (Figure 5).The introduction of linkage can effectively improve the weak interfacial contact and overcome the long distance of electron transport between the graphene and TiO2, leading to raised separation and mobility of photoinduced electrons and holes and thus higher photocatalytic activity.Despite of electron accepter and electron storage, graphene can also act as a transport bridge between photocatalysts.For example, in the 2D ternary BiVO4/graphene oxide (GO)/TiO2 system, both the BiVO4 and the TiO2 were connected to GO forming a p-n heterogeneous structure.The CB of BiVO4 was more negative than that of GO and the CB of GO was more negative than that of TiO2; thus, the electrons generated from the CB of BiVO4 can transfer to the GO and then the electron further moved to the conduction band of TiO2 (Figure 6).Therefore, the GO can enhance the effective separation of the photo-generated electron-hole pairs due to its superior electrical conductivity.Meanwhile, the large surface area of the GO is also beneficial for dye attachment [74].Except for dimension factor and bonding interaction between graphene and TiO 2 , a linkage is introduced to graphene/TiO 2 system to achieve better interfacial contact as well.A N-doping Graphene-TiO 2 composite nano-capsule for gaseous HCHO degradation was reported [58].It indicated that wrapping with dopamine on the surface of TiO 2 enhanced interfacial contact between TiO 2 and melamine-doped graphene (MG) sheets, thus promoting the separation and mobility of photoinduced electrons and holes in TiO 2 @MG-D.The dopamine acted as bridge between TiO 2 and MG, creating numerous migration channels for charges and restraining the recombination of electrons and holes (Figure 5).The introduction of linkage can effectively improve the weak interfacial contact and overcome the long distance of electron transport between the graphene and TiO 2 , leading to raised separation and mobility of photoinduced electrons and holes and thus higher photocatalytic activity.Except for dimension factor and bonding interaction between graphene and TiO2, a linkage is introduced to graphene/TiO2 system to achieve better interfacial contact as well.A N-doping Graphene-TiO2 composite nano-capsule for gaseous HCHO degradation was reported [58].It indicated that wrapping with dopamine on the surface of TiO2 enhanced interfacial contact between TiO2 and melamine-doped graphene (MG) sheets, thus promoting the separation and mobility of photoinduced electrons and holes in TiO2@MG-D.The dopamine acted as bridge between TiO2 and MG, creating numerous migration channels for charges and restraining the recombination of electrons and holes (Figure 5).The introduction of linkage can effectively improve the weak interfacial contact and overcome the long distance of electron transport between the graphene and TiO2, leading to raised separation and mobility of photoinduced electrons and holes and thus higher photocatalytic activity.Despite of electron accepter and electron storage, graphene can also act as a transport bridge between photocatalysts.For example, in the 2D ternary BiVO4/graphene oxide (GO)/TiO2 system, both the BiVO4 and the TiO2 were connected to GO forming a p-n heterogeneous structure.The CB of BiVO4 was more negative than that of GO and the CB of GO was more negative than that of TiO2; thus, the electrons generated from the CB of BiVO4 can transfer to the GO and then the electron further moved to the conduction band of TiO2 (Figure 6).Therefore, the GO can enhance the effective separation of the photo-generated electron-hole pairs due to its superior electrical conductivity.Meanwhile, the large surface area of the GO is also beneficial for dye attachment [74].Despite of electron accepter and electron storage, graphene can also act as a transport bridge between photocatalysts.For example, in the 2D ternary BiVO 4 /graphene oxide (GO)/TiO 2 system, both the BiVO 4 and the TiO 2 were connected to GO forming a p-n heterogeneous structure.The CB of BiVO 4 was more negative than that of GO and the CB of GO was more negative than that of TiO 2 ; thus, the electrons generated from the CB of BiVO4 can transfer to the GO and then the electron further moved to the conduction band of TiO 2 (Figure 6).Therefore, the GO can enhance the effective separation of the photo-generated electron-hole pairs due to its superior electrical conductivity.Meanwhile, the large surface area of the GO is also beneficial for dye attachment [74].

Graphdiyne Modified TiO2
Graphdiyne (GD) is a new carbon allotrope in which the benzene rings are conjugated by 1,3diyne bonds to form a 2D planar network structure and features both sp and sp 2 carbon atoms.Since the successful synthesis by Li et al. [75], GD has evoked significant interest in various scientific fields because of unique mechanical, chemical and electrical properties [38,42,[76][77][78][79][80].GD shows potential for photocatalysis with its large surface area as well as high charge mobility.GD features an intrinsic bandgap and exhibits semiconducting property with a measured conductivity of 2.516 × 10 −4 S•m −1 and was predicted to be the most stable structure among various diacetylenic non-natural carbon allotropes [81].It also provides highly active sites for catalysis.Furthermore, GD with diacetylene linkage can be chemically bonded with TiO2 [82][83][84][85].Therefore, the TiO2-graphdiyne composites can greatly improve the photocatalytic activity, and thus their application in photocatalysis has been explored recently [83,84,86].

The Synthesis of GD/TiO2 Composites
The general preparation of GD film is through a coupling reaction in which hexaethynylbenzene (HEB) acts as precursor and copper foil serves as catalysis.Meanwhile, the copper foil provides a large planar substrate for the directional polymerization growth of the GD film (Figure 7).Despite of film, GD with different morphologies such as nanotube arrays, nanowires, nanowalls and nanosheets have been also prepared for diverse applications [87,88].

Graphdiyne Modified TiO 2
Graphdiyne (GD) is a new carbon allotrope in which the benzene rings are conjugated by 1,3-diyne bonds to form a 2D planar network structure and features both sp and sp 2 carbon atoms.Since the successful synthesis by Li et al. [75], GD has evoked significant interest in various scientific fields because of unique mechanical, chemical and electrical properties [38,42,[76][77][78][79][80].GD shows potential for photocatalysis with its large surface area as well as high charge mobility.GD features an intrinsic bandgap and exhibits semiconducting property with a measured conductivity of 2.516 × 10 −4 S•m −1 and was predicted to be the most stable structure among various diacetylenic non-natural carbon allotropes [81].It also provides highly active sites for catalysis.Furthermore, GD with diacetylene linkage can be chemically bonded with TiO 2 [82][83][84][85].Therefore, the TiO 2 -graphdiyne composites can greatly improve the photocatalytic activity, and thus their application in photocatalysis has been explored recently [83,84,86].

The Synthesis of GD/TiO 2 Composites
The general preparation of GD film is through a coupling reaction in which hexaethynylbenzene (HEB) acts as precursor and copper foil serves as catalysis.Meanwhile, the copper foil provides a large planar substrate for the directional polymerization growth of the GD film (Figure 7).Despite of film, GD with different morphologies such as nanotube arrays, nanowires, nanowalls and nanosheets have been also prepared for diverse applications [87,88].

Graphdiyne Modified TiO2
Graphdiyne (GD) is a new carbon allotrope in which the benzene rings are conjugated by 1,3diyne bonds to form a 2D planar network structure and features both sp and sp 2 carbon atoms.Since the successful synthesis by Li et al. [75], GD has evoked significant interest in various scientific fields because of unique mechanical, chemical and electrical properties [38,42,[76][77][78][79][80].GD shows potential for photocatalysis with its large surface area as well as high charge mobility.GD features an intrinsic bandgap and exhibits semiconducting property with a measured conductivity of 2.516 × 10 −4 S•m −1 and was predicted to be the most stable structure among various diacetylenic non-natural carbon allotropes [81].It also provides highly active sites for catalysis.Furthermore, GD with diacetylene linkage can be chemically bonded with TiO2 [82][83][84][85].Therefore, the TiO2-graphdiyne composites can greatly improve the photocatalytic activity, and thus their application in photocatalysis has been explored recently [83,84,86].

The Synthesis of GD/TiO2 Composites
The general preparation of GD film is through a coupling reaction in which hexaethynylbenzene (HEB) acts as precursor and copper foil serves as catalysis.Meanwhile, the copper foil provides a large planar substrate for the directional polymerization growth of the GD film (Figure 7).Despite of film, GD with different morphologies such as nanotube arrays, nanowires, nanowalls and nanosheets have been also prepared for diverse applications [87,88].Ex-situ hydrothermal method is commonly used in preparation of GD/TiO 2 composites [83,84,86].In general, the GD and TiO 2 are prepared separately.Then the pre-prepared GD and TiO 2 are mixed in H 2 O/CH 3 OH solvent.After stirring to obtain a homogeneous suspension, the suspension is placed in Teflon sealed autoclave and heated to combine the TiO 2 and GD.Being rinsed and dried, the GD/TiO 2 composites are obtained.

The Role of GD in TiO 2 Photocatalysis
Wang et al. [84] were the first to combine GD with TiO 2 for the enhancement of TiO 2 photocatalysis.The resultant GD-P25 composites exhibited higher visible light photocatalytic activity than those of the bare P25, P25-CNT (titania-carbon nanotube), and P25-GR (graphene) materials.By changing the weight percent of GD in the hybrid, the photocatalytic activity of P25-GD can be adjusted.It was speculated that the formation of chemical bonds between P25 and GD can effectively decrease the bandgap of P25 and extended its absorbable light range [84].Namely, electrons in VB of TiO 2 can easily migrate to impurity band which is attributed to the insertion of carbon p-orbitals into the TiO 2 bandgap, and then transfer to CB of TiO 2 thus enhancing the photo-response activity.In order to further explore the role of GD, Yang et al. [83] investigated the chemical structures and electronic properties of TiO 2 -GD and TiO 2 -GR composites employing first-principles density functional theory (DFT) calculations.The results revealed that for the TiO 2 (001)-GR composite, O and atop C atoms could form C-O σ bond, which acted as a charge transfer bridge at the interface between TiO 2 and GR.Besides the C-O σ bond, another Ti-C π bond is also formed in TiO 2 (001)-GD composite, which makes GD combine with TiO 2 tightly and therefore enhances the charge transfer.In addition, calculated Mulliken charge for the surface of TiO 2 (001)-GD and TiO 2 (001)-GR suggested a stronger electrons' capture ability of former (Figure 8).The calculated results were in accordance with theoretical prediction that TiO 2 (001)-GD composites showed the highest photocatalysis performance among 2D carbon-based TiO 2 composites, confirming that GD could become a promising competitor in the field of photocatalysis.After that, Dong et al. prepared GD-hybridized nitrogen-doped TiO 2 nanosheets with exposed (001) facets (GD-NTNS) [86].The doped N and incorporated GD efficiently narrowed the bandgap compared with pure TiO 2 and widened response range towards light from UV light to 420 nm visible light.The activity of the GD-NTNS photocatalyst presented the most superior performance compared with bare TiO 2 nanosheets (TNS) and nitrogen-doped TiO 2 nanosheets (NTNS) and GR-NTNS.
Catalysts 2018, 8, x FOR PEER REVIEW 8 of 25 Ex-situ hydrothermal method is commonly used in preparation of GD/TiO2 composites [83,84,86].In general, the GD and TiO2 are prepared separately.Then the pre-prepared GD and TiO2 are mixed in H2O/CH3OH solvent.After stirring to obtain a homogeneous suspension, the suspension is placed in Teflon sealed autoclave and heated to combine the TiO2 and GD.Being rinsed and dried, the GD/TiO2 composites are obtained.

The Role of GD in TiO2 Photocatalysis
Wang et al. [84] were the first to combine GD with TiO2 for the enhancement of TiO2 photocatalysis.The resultant GD-P25 composites exhibited higher visible light photocatalytic activity than those of the bare P25, P25-CNT (titania-carbon nanotube), and P25-GR (graphene) materials.By changing the weight percent of GD in the hybrid, the photocatalytic activity of P25-GD can be adjusted.It was speculated that the formation of chemical bonds between P25 and GD can effectively decrease the bandgap of P25 and extended its absorbable light range [84].Namely, electrons in VB of TiO2 can easily migrate to impurity band which is attributed to the insertion of carbon p-orbitals into the TiO2 bandgap, and then transfer to CB of TiO2 thus enhancing the photo-response activity.In order to further explore the role of GD, Yang et al. [83] investigated the chemical structures and electronic properties of TiO2-GD and TiO2-GR composites employing first-principles density functional theory (DFT) calculations.The results revealed that for the TiO2 (001)-GR composite, O and atop C atoms could form C-O σ bond, which acted as a charge transfer bridge at the interface between TiO2 and GR.Besides the C-O σ bond, another Ti-C π bond is also formed in TiO2 (001)-GD composite, which makes GD combine with TiO2 tightly and therefore enhances the charge transfer.In addition, calculated Mulliken charge for the surface of TiO2 (001)-GD and TiO2 (001)-GR suggested a stronger electrons' capture ability of former (Figure 8).The calculated results were in accordance with theoretical prediction that TiO2 (001)-GD composites showed the highest photocatalysis performance among 2D carbon-based TiO2 composites, confirming that GD could become a promising competitor in the field of photocatalysis.After that, Dong et al. prepared GD-hybridized nitrogen-doped TiO2 nanosheets with exposed (001) facets (GD-NTNS) [86].The doped N and incorporated GD efficiently narrowed the bandgap compared with pure TiO2 and widened response range towards light from UV light to 420 nm visible light.The activity of the GD-NTNS photocatalyst presented the most superior performance compared with bare TiO2 nanosheets (TNS) and nitrogen-doped TiO2 nanosheets (NTNS) and GR-NTNS.The mechanisms of photocatalysis enhancement by introducing GD remain to be understood.In general, with a lower Fermi level than the conduction band minimum of TiO2, GD can be regarded as an electron pool which accept electrons excited from TiO2 [84,89,90] (Figure 9).As a result, it prompts the charge carriers' separation and prevents electron-hole recombination.Moreover, GD can generate an impurity band and thus broaden the visible light absorption in TiO2-GD composites [91][92][93].The mechanisms of photocatalysis enhancement by introducing GD remain to be understood.In general, with a lower Fermi level than the conduction band minimum of TiO 2 , GD can be regarded as an electron pool which accept electrons excited from TiO 2 [84,89,90] (Figure 9).As a result, it prompts the charge carriers' separation and prevents electron-hole recombination.Moreover, GD can generate an impurity band and thus broaden the visible light absorption in TiO 2 -GD composites [91][92][93].

C3N4 Modified TiO2
Graphitic carbon nitride (g-C3N4) is a 2D polymer material which shows broad application prospects in many fields, given the simple synthesis, rich source, along with unique electronic structure, good thermal stability and chemical stability.Its graphene-like structure is composed of triazine (C3N3) or tri-s-striazine (C6N7) allotropes units (Figure 10).The tri-s-striazine unit structure is more stable and thus draws in extensive studies [34].Since the first report of g-C3N4 for water decomposition, g-C3N4 has attracted wide attention in photocatalyst [40].The bandgap of g-C3N4 (2.6-2.7 eV) is moderate and the substantial nitrogen sites and ordered units structure endue g-C3N4 an ideal material to composite with TiO2.

The Synthesis of g-C3N4/TiO2 Composites
In general, the synthesis of g-C3N4/TiO2 composites can be also divided into ex-situ method and in-situ method. In the ex-situ way, both g-C3N4 and TiO2 materials are pre-prepared, which can be integrated through physical process such as ball milling [94], solvent evaporation [95,96], etc.Though physical process is easy to operate under moderate conditions, some flaws also exist such as ununiformly dispersing and unstable structure. The in-situ method uses one of the materials as a substrate and then the other material grows on the surface of the substrate.For g-C3N4/TiO2 composites, both materials can be regarded as substrates.

C 3 N 4 Modified TiO 2
Graphitic carbon nitride (g-C 3 N 4 ) is a 2D polymer material which shows broad application prospects in many fields, given the simple synthesis, rich source, along with unique electronic structure, good thermal stability and chemical stability.Its graphene-like structure is composed of triazine (C 3 N 3 ) or tri-s-striazine (C 6 N 7 ) allotropes units (Figure 10).The tri-s-striazine unit structure is more stable and thus draws in extensive studies [34].Since the first report of g-C 3 N 4 for water decomposition, g-C 3 N 4 has attracted wide attention in photocatalyst [40].The bandgap of g-C 3 N 4 (2.6-2.7 eV) is moderate and the substantial nitrogen sites and ordered units structure endue g-C 3 N 4 an ideal material to composite with TiO 2 .

C3N4 Modified TiO2
Graphitic carbon nitride (g-C3N4) is a 2D polymer material which shows broad application prospects in many fields, given the simple synthesis, rich source, along with unique electronic structure, good thermal stability and chemical stability.Its graphene-like structure is composed of triazine (C3N3) or tri-s-striazine (C6N7) allotropes units (Figure 10).The tri-s-striazine unit structure is more stable and thus draws in extensive studies [34].Since the first report of g-C3N4 for water decomposition, g-C3N4 has attracted wide attention in photocatalyst [40].The bandgap of g-C3N4 (2.6-2.7 eV) is moderate and the substantial nitrogen sites and ordered units structure endue g-C3N4 an ideal material to composite with TiO2.

The Synthesis of g-C3N4/TiO2 Composites
In general, the synthesis of g-C3N4/TiO2 composites can be also divided into ex-situ method and in-situ method. In the ex-situ way, both g-C3N4 and TiO2 materials are pre-prepared, which can be integrated through physical process such as ball milling [94], solvent evaporation [95,96], etc.Though physical process is easy to operate under moderate conditions, some flaws also exist such as ununiformly dispersing and unstable structure. The in-situ method uses one of the materials as a substrate and then the other material grows on the surface of the substrate.For g-C3N4/TiO2 composites, both materials can be regarded as substrates.

The Synthesis of g-C 3 N 4 /TiO 2 Composites
In general, the synthesis of g-C 3 N 4 /TiO 2 composites can be also divided into ex-situ method and in-situ method.

•
In the ex-situ way, both g-C 3 N 4 and TiO 2 materials are pre-prepared, which can be integrated through physical process such as ball milling [94], solvent evaporation [95,96], etc.Though physical process is easy to operate under moderate conditions, some flaws also exist such as ununiformly dispersing and unstable structure.

•
The in-situ method uses one of the materials as a substrate and then the other material grows on the surface of the substrate.For g-C 3 N 4 /TiO 2 composites, both materials can be regarded as substrates.
• When used as substrates, g-C 3 N 4 is pre-prepared by calcinations of precursors.Solvothermal/ hydrothermal method is most common for the next step.After mixing g-C 3 N 4 and titanates in a certain solvent, the solution is well dispersed and sealed in the Teflon-lined autoclave, followed by a solvothermal/hydrothermal treatment [97][98][99].Furthermore, Atomic Layer Deposition (ALD) was applied to form thin TiO 2 films on g-C 3 N 4 substrates.ALD involves the surface of a substrate exposed alternately to alternating precursor flow.Then the precursor molecule reacts with the surface in a self-limiting way, which guarantees that the reaction stops as all the reactive sites on the substrate reacted with the precursors.It is an effective way to control the thickness and homogeneity of deposited layer [100].

•
When TiO 2 was used as substrates, calcination is widely used for the convenience and easy operation.In this process, the solid mixture of TiO 2 and pure urea or melamine or dicyandiamide powder are calcinated under fixed temperature to obtain g-C 3 N 4 /TiO 2 composites.Before calcination, the two components should be evenly dispersed by sonication [101], stirring [102], or grounding [103].Recently, Tan et al. [104] reported another facile one-step way to prepare nanostructured g-C 3 N 4 /TiO 2 composite.As seen in Figure 11, melamine was at the bottom of the crucible while P25 was on the top of a cylinder put in the crucible.After a 4-h vapor deposition process, nanostructured g-C 3 N 4 /TiO 2 composite was obtained.
Catalysts 2018, 8, x FOR PEER REVIEW 10 of 25  When used as substrates, g-C3N4 is pre-prepared by calcinations of precursors.Solvothermal/hydrothermal method is most common for the next step.After mixing g-C3N4 and titanates in a certain solvent, the solution is well dispersed and sealed in the Teflonlined autoclave, followed by a solvothermal/hydrothermal treatment [97][98][99].Furthermore, Atomic Layer Deposition (ALD) was applied to form thin TiO2 films on g-C3N4 substrates.ALD involves the surface of a substrate exposed alternately to alternating precursor flow.
Then the precursor molecule reacts with the surface in a self-limiting way, which guarantees that the reaction stops as all the reactive sites on the substrate reacted with the precursors.It is an effective way to control the thickness and homogeneity of deposited layer [100]. When TiO2 was used as substrates, calcination is widely used for the convenience and easy operation.In this process, the solid mixture of TiO2 and pure urea or melamine or dicyandiamide powder are calcinated under fixed temperature to obtain g-C3N4/TiO2 composites.Before calcination, the two components should be evenly dispersed by sonication [101], stirring [102], or grounding [103].Recently, Tan et al. [104] reported another facile one-step way to prepare nanostructured g-C3N4/TiO2 composite.As seen in Figure 11, melamine was at the bottom of the crucible while P25 was on the top of a cylinder put in the crucible.After a 4-h vapor deposition process, nanostructured g-C3N4/TiO2 composite was obtained.

The Role of g-C3N4 in Photocatalysis
With a moderate bandgap of ~2.7 eV, g-C3N4 shows ability of photocatalyst under visible light, in contrast to TiO2, which owns a large bandgap of 3.2 eV (Figure 12).However, because of the rapid recombination of photogenerated electron-hole pairs, the synergistic effect between g-C3N4 and TiO2 plays important roles.In a photocatalyst system of g-C3N4/TiO2 composites, the CB electrons of g-C3N4 transfer to the CB of TiO2 and the VB holes of TiO2 transfer to the VB of g-C3N4, which is a typical Type II system [41].The electron/hole conduction mechanism can effectively separate electrons and holes, and thus enhances the separation efficiency and inhibit the recombination.

The Role of g-C 3 N 4 in Photocatalysis
With a moderate bandgap of ~2.7 eV, g-C 3 N 4 shows ability of photocatalyst under visible light, in contrast to TiO 2 , which owns a large bandgap of 3.2 eV (Figure 12).However, because of the rapid recombination of photogenerated electron-hole pairs, the synergistic effect between g-C 3 N 4 and TiO 2 plays important roles.In a photocatalyst system of g-C 3 N 4 /TiO 2 composites, the CB electrons of g-C 3 N 4 transfer to the CB of TiO 2 and the VB holes of TiO 2 transfer to the VB of g-C 3 N 4 , which is a typical Type II system [41].The electron/hole conduction mechanism can effectively separate electrons and holes, and thus enhances the separation efficiency and inhibit the recombination.
Catalysts 2018, 8, x FOR PEER REVIEW 10 of 25  When used as substrates, g-C3N4 is pre-prepared by calcinations of precursors.Solvothermal/hydrothermal method is most common for the next step.After mixing g-C3N4 and titanates in a certain solvent, the solution is well dispersed and sealed in the Teflonlined autoclave, followed by a solvothermal/hydrothermal treatment [97][98][99].Furthermore, Atomic Layer Deposition (ALD) was applied to form thin TiO2 films on g-C3N4 substrates.ALD involves the surface of a substrate exposed alternately to alternating precursor flow.
Then the precursor molecule reacts with the surface in a self-limiting way, which guarantees that the reaction stops as all the reactive sites on the substrate reacted with the precursors.It is an effective way to control the thickness and homogeneity of deposited layer [100]. When TiO2 was used as substrates, calcination is widely used for the convenience and easy operation.In this process, the solid mixture of TiO2 and pure urea or melamine or dicyandiamide powder are calcinated under fixed temperature to obtain g-C3N4/TiO2 composites.Before calcination, the two components should be evenly dispersed by sonication [101], stirring [102], or grounding [103].Recently, Tan et al. [104] reported another facile one-step way to prepare nanostructured g-C3N4/TiO2 composite.As seen in Figure 11, melamine was at the bottom of the crucible while P25 was on the top of a cylinder put in the crucible.After a 4-h vapor deposition process, nanostructured g-C3N4/TiO2 composite was obtained.

The Role of g-C3N4 in Photocatalysis
With a moderate bandgap of ~2.7 eV, g-C3N4 shows ability of photocatalyst under visible light, in contrast to TiO2, which owns a large bandgap of 3.2 eV (Figure 12).However, because of the rapid recombination of photogenerated electron-hole pairs, the synergistic effect between g-C3N4 and TiO2 plays important roles.In a photocatalyst system of g-C3N4/TiO2 composites, the CB electrons of g-C3N4 transfer to the CB of TiO2 and the VB holes of TiO2 transfer to the VB of g-C3N4, which is a typical Type II system [41].The electron/hole conduction mechanism can effectively separate electrons and holes, and thus enhances the separation efficiency and inhibit the recombination.The structure plays a vital role in enhancing photocatalysis efficiency.g-C 3 N 4 nanosheets (NS)-TiO 2 mesocrystals (TMC) composites was prepared by in-situ process [105].Compared with bulk g-C 3 N 4 /TMC composites, the H 2 evolution rate of g-C 3 N 4 (NS)/TMC was about six times higher, which was possibly due to a larger surface area of g-C 3 N 4 (NS)/TMC (57.4 m 2 g −1 ) than that of bulk g-C 3 N 4 /TMC (34.3 m 2 g −1 ).What's more, the g-C 3 N 4 nanosheets owned a lower surface defect density, given the surface defects normally is seen as recombination centers for photoinduced electrons and holes.However, surface area is not the unparalleled factor of promoted efficiency of photocatalyst, taking the fact that the surface area of g-C 3 N 4 NS (31 wt%)/TMC (57.4 m 2 g −1 ) and g-C 3 N 4 NS (31 wt%)/P25 (52.3 m 2 g −1 ) was nearly the same, as the H 2 evolution rate of g-C 3 N 4 (NS)/TMC was about 7 times higher.Further research indicated that the tight interface between g-C 3 N 4 NS and TMC facilitated the charge transfer, which is a flexible way to promote solar energy utilization of g-C 3 N 4 /TiO 2 photocatalyst.
Other structures like core-shell was lucubrated to create high photocatalytic activity towards many dyes [106].After in-situ calcination and growth of cyanamide on the surface of TiO 2 , a multiple direction contact structure of TiO 2 @g-C 3 N 4 hollow core@shell heterojunction photocatalyst (HTCN-1) was synthesized.The g-C 3 N 4 nanosheets grew on the surface of TiO 2 caused closer contact between TiO 2 and g-C 3 N 4 and a larger interfacial area, as confirmed by XPS analysis [106].Compared with another core-shell type TiO 2 @g-C 3 N 4 (C-T) with unidirectional contact structures [107], HTCN-1 possessed higher efficiency in the charge separation and enhanced charge transfer.It demonstrated that multiple direction contact resulted in a large interfacial area, which would provide sufficient channels for efficient and rapid charge transfer (Figure 13) [106].In another core-shell structure of g-C 3 N 4 /TiO 2 hybrid, Ag was introduced as interlayers to participate in electrical conduction and bridge the gap between g-C 3 N 4 and TiO 2 , facilitating the separation of photoexcited charge and reducing the recombination of the photogenerated electron hole (Figure 14) [108].The surface area of the samples didn't change much upon the introduction of Ag (228.4 m 2 g −1 and 210.3 m 2 g −1 for Ag/TiO 2 microspheres and nonsilver containing TiO 2 , respectively).It was worth noting that low content of g-C 3 N 4 (2%) in g-C 3 N 4 /Ag/TiO 2 microspheres had a larger surface area but lower photocatalytic activity than the g-C 3 N 4 (4%)/Ag/TiO 2 microsphere sample [108].The possible reason was that high content of g-C 3 N 4 can generate more electron-hole pairs, leading to a higher photocatalytic activity.However, the g-C 3 N 4 (6%)/Ag/TiO 2 microsphere sample showed decreased photocatalytic activity due to reduced surface area, which limited the contact between the catalyst and pollutant and thus lowered the photocatalytic reaction.It reflects that proper surface area is needed to provide both active sites and reaction sites.
The doping of g-C 3 N 4 is another viable way to realize structure modification process.Sulfur was introduced to g-C 3 N 4 nanostructures, and their photocatalytic performance was studied for decomposition of MO dye under visible light.The degradation efficiency over g-C 3 N 4 -TiO 2 composites (CNT) reached 61% within 90 min, while S-C 3 N 4 -TiO 2 composites (SCNT) reached nearly 100% within the same period [109].SEM image showed a more transparent and thinner layer of S-C 3 N 4 compared with g-C 3 N 4 when composited with TiO 2 , leading to an enhanced visible light absorption capability.On the other hand, unique bar-like structure of SCNT provided a pathway for carriers and isolate photon absorption with carriers' collection in perpendicular directions.Meanwhile, TiO 2 nanoparticles were more evenly dispersed on and inside S-C 3 N 4 substrate in SCNT sample, which is beneficial for the interfacial carriers' transportation between S-C 3 N 4 layer and TiO 2 particle [109].Calculations revealed that the modified electronic structure with elevation of CB and VB values owing to doped sulfur, contributed to a higher driving force from CB of S-C 3 N 4 to CB of TiO 2 and thus promoted the separation efficiency of electron-hole pairs (Figure 15).The doping of sulfur alternated both the structure and level distribution of C 3 N 4 , causing excellent separation efficiency of electron-hole pair when contacted with TiO 2 .

MoS 2 Modified TiO 2
2D layered transition metal chalcogenides (TMCs) nanostructures spark a research boom due to its unique physical and chemical properties compared with other 2D materials.The usual formula of TMCs is MX 2 , while M is transition metal and X is chalcogenide element, namely, S, Se, or Te.Because of the typical 2D structure with high surface-to-volume ratio and missing coordination at edge (Figure 16), TMCs exhibits high chemical sensitivity [36].Considering its versatile physicochemical properties, TMCs can be applied in catalyst [41], energy storage [39], and biology [110].Some TMCs such as WS 2 [111], TiS 2 [112] are also used in TiO 2 photocatalysis.Among TMCs, MoS 2 show extraordinary potential as semiconductors owing to its thickness dependent bandgap and natural abundance.When bulk MoS 2 are stripped into a single layer or several layers of nanosheets, the indirect bandgap (1.3 eV) can be converted to a direct bandgap (1.8 eV) [113] and show excellent performance in photocatalysis after compositing with TiO 2 [114].Besides, its high surface-to-volume ratio makes up for the limitation of the low theoretical specific capacity of TiO 2 .The synergy between MoS 2 and TiO 2 endows the TiO 2 /MoS 2 composite superior performance compared to their single material.

MoS2 Modified TiO2
2D layered transition metal chalcogenides (TMCs) nanostructures spark a research boom due to its unique physical and chemical properties compared with other 2D materials.The usual formula of TMCs is MX2, while M is transition metal and X is chalcogenide element, namely, S, Se, or Te.Because of the typical 2D structure with high surface-to-volume ratio and missing coordination at edge (Figure 16), TMCs exhibits high chemical sensitivity [36].Considering its versatile physicochemical properties, TMCs can be applied in catalyst [41], energy storage [39], and biology [110].Some TMCs such as WS2 [111], TiS2 [112] are also used in TiO2 photocatalysis.Among TMCs, MoS2 show extraordinary potential as semiconductors owing to its thickness dependent bandgap and natural abundance.When bulk MoS2 are stripped into a single layer or several layers of nanosheets, the indirect bandgap (1.3 eV) can be converted to a direct bandgap (1.8 eV) [113] and show excellent performance in photocatalysis after compositing with TiO2 [114].Besides, its high surface-to-volume ratio makes up for the limitation of the low theoretical specific capacity of TiO2.The synergy between MoS2 and TiO2 endows the TiO2/MoS2 composite superior performance compared to their single material.Similar to the synthesis methods of graphene/TiO 2 composite, the synthesis of MoS 2 /TiO 2 composites is also divided into ex-situ methods and in-situ methods.For the in-situ method, TiO 2 and MoS 2 are synthesized separately, then the two are combined by various methods, such as hydrothermal/solvothermal assembly [115,116], mechanical method [117], drop-casting [118], or sol-gel [119], which can be also applied for in-situ methods [120,121].The ex-situ method is simple and inexpensive, but the two compounds have poor dispersion and show weak interactions.Despite the same process as ex-situ method, there are chemical vapor deposition [122] and co-reduction precipitation [123] in in-situ process.Among them, the hydrothermal method is simple, easy to operate, and has good controllability, and thus is most commonly used in the preparation of MoS 2 /TiO 2 composite materials.The in-situ reduction method uses one of the materials as a substrate and then coats or loads the other material.This involves the molybdenum disulfide as substrate or TiO 2 as a substrate.The following paragraphs will discuss the two kinds of composites.

•
MoS 2 as substrate.In this process, MoS 2 are pre-prepared as substrate for the in-situ growth of TiO 2 .Hydrothermal method is widely used in which tetrabutyl titanate serves as titanate source [124,125].Recently, another approach has been developed to synthesize MoS 2 @TiO 2 composites.Ren et al. [126] reported TiO 2 -modified MoS 2 nanosheet arrays by the ALD process, coating a thin layer of TiO 2 on both the edge and basal planes of TiO 2 (Figure 17).It provides a new insight for the combination of sites at the basal planes of TiO 2 .

•
TiO 2 composite as substrate.For coated MoS 2 /TiO 2 composites, TiO 2 are usually substrates.Liu et al. [127] reported a N-TiO 2-x @MoS 2 core-shell heterostructure composite.TBT and urea were used to prepare N-doped TiO 2 microspheres (N-TiO 2 ) with a smooth surface by hydrothermal method.Considering the growth of molybdenum sulfide on the TiO 2 substrate, specific morphology and growth sites of TiO 2 is needed.Sun et al. [128] took a targeted etching route to control the morphology of TiO 2 /MoS 2 nanocomposites.Hollow microspheres structured TiO 2 /MoS 2 showed a higher dye degradation activity due to a larger proportion of interface, compared to TiO 2 /MoS 2 nanocomposites of yolk-shell structures.Other structures such as nanobelts and nanotubes have also been developed [129,130].In addition to the morphology, the formation of a specific crystal structure of TiO 2 as a substrate has also got attention to prepare high performance MoS 2 /TiO 2 composites [130,131].He et al. [130] reported a few-layered 1T-MoS 2 coating on Si doped TiO 2 nanotubes (MoS 2 /TiO 2 NTs hybrids) through hydrothermal process.Because of the higher catalytic activity of 1T phase of MoS 2 and Si doped TiO 2 , MoS 2 /TiO 2 NTs hybrids nanocomposites exhibited excellent photocatalytic activity.
Catalysts 2018, 8, x FOR PEER REVIEW 14 of 25 hydrothermal/solvothermal assembly [115,116], mechanical method [117], drop-casting [118], or solgel [119], which can be also applied for in situ methods [120,121].The ex situ method is simple and inexpensive, but the two compounds have poor dispersion and show weak interactions.Despite the same process as ex situ method, there are chemical vapor deposition [122] and co-reduction precipitation [123] in in situ process.Among them, the hydrothermal method is simple, easy to operate, and has good controllability, and thus is most commonly used in the preparation of MoS2/TiO2 composite materials.The in situ reduction method uses one of the materials as a substrate and then coats or loads the other material.This involves the molybdenum disulfide as substrate or TiO2 as a substrate.The following paragraphs will discuss the two kinds of composites.
 MoS2 as substrate.In this process, MoS2 are pre-prepared as substrate for the in situ growth of TiO2.Hydrothermal method is widely used in which tetrabutyl titanate serves as titanate source [124,125].Recently, another approach has been developed to synthesize MoS2@TiO2 composites.Ren et al. [126] reported TiO2-modified MoS2 nanosheet arrays by the ALD process, coating a thin layer of TiO2 on both the edge and basal planes of TiO2 (Figure 17).It provides a new insight for the combination of sites at the basal planes of TiO2. TiO2 composite as substrate.For coated MoS2/TiO2 composites, TiO2 are usually substrates.Liu et al. [127] reported a N-TiO2-x@MoS2 core-shell heterostructure composite.TBT and urea were used to prepare N-doped TiO2 microspheres (N-TiO2) with a smooth surface by hydrothermal method.Considering the growth of molybdenum sulfide on the TiO2 substrate, specific morphology and growth sites of TiO2 is needed.Sun et al. [128] took a targeted etching route to control the morphology of TiO2/MoS2 nanocomposites.Hollow microspheres structured TiO2/MoS2 showed a higher dye degradation activity due to a larger proportion of interface, compared to TiO2/MoS2 nanocomposites of yolk-shell structures.Other structures such as nanobelts and nanotubes have also been developed [129,130].In addition to the morphology, the formation of a specific crystal structure of TiO2 as a substrate has also got attention to prepare high performance MoS2/TiO2 composites [130,131].He et al. [130] reported a few-layered 1T-MoS2 coating on Si doped TiO2 nanotubes (MoS2/TiO2 NTs hybrids) through hydrothermal process.Because of the higher catalytic activity of 1T phase of MoS2 and Si doped TiO2, MoS2/TiO2 NTs hybrids nanocomposites exhibited excellent photocatalytic activity.Reprinted with permission from [126].Copyright 2017, Wiley-VCH.

The Role of MoS2 in TiO2 Photocatalysis
During the photocatalysis process, electrons transfer through the interface between TiO2 and MoS2, and therefore the contact between the two is vital for photocatalytic activity.A strategy for construction of 3D semiconductor heterojunction structure by TiO2 and 2D-structured MoS2 is proposed to achieve increase of active sites and decrease of electron-hole pair combination [127,132].For example, a 3D flower-like N-TiO2-x@MoS2 was obtained by hydrothermal method.Considering that the smooth TiO2 nanosphere shows poor affinity when coated with MoS2 nanosheets, TiO2 was doped with N and Ti 3+ .X-ray photoelectron spectroscopy (XPS) shows the existence of electronic

The Role of MoS 2 in TiO 2 Photocatalysis
During the photocatalysis process, electrons transfer through the interface between TiO 2 and MoS 2 , and therefore the contact between the two is vital for photocatalytic activity.A strategy for construction of 3D semiconductor heterojunction structure by TiO 2 and 2D-structured MoS 2 is proposed to achieve increase of active sites and decrease of electron-hole pair combination [127,132].For example, a 3D flower-like N-TiO 2-x @MoS 2 was obtained by hydrothermal method.Considering that the smooth TiO 2 nanosphere shows poor affinity when coated with MoS 2 nanosheets, TiO 2 was doped with N and Ti 3+ .X-ray photoelectron spectroscopy (XPS) shows the existence of electronic interactions between MoS 2 and N-TiO 2-x and the strong heterostructure effect between the MoS 2 nanoflower and N-TiO 2-x nanosphere [127].Another study of 3D TiO 2 @MoS 2 revealed that the formation of Ti-S bonds made TiO 2 nanoarrays firmly grasp MoS 2 , thus affording a marvelous mechanical stability for the integrated architectures [133].
Different phase of MoS 2 exhibits various chemical and physical properties when combined with TiO 2 .MoS 2 has two main phases, namely the metallic 1T phase and semiconducting 2H phase.As for 2H phase, the active site with catalytic activity is located at the edge of the MoS 2 layers and the basal surface of MoS 2 is catalytically inactive [134].Therefore, the 1T phase of MoS 2 with active sites on both edge and basal planes attracts researchers' attention in recent years [118,125,131].A typical schematic of MoS 2 /TiO 2 composites for photocatalytic hydrogen production is shown in Figure 18.The 1T-MoS 2 nanosheets not only provide extra reaction sites on the basal plane, but also play a role in electron delivery.Because of the active site distributing on the edge of 2H-MoS 2 nanosheets, the photogenerated electron from TiO 2 needs a long-distance move before reacted with H 2 O.This leaded to a lower diffusion rate compared with 1T-MoS 2 /TiO 2 composites and thus enhanced the separation efficiency of electron-hole pairs.Therefore, the 1T-MoS 2 /TiO 2 composites exhibited excellent photocatalytic activity as the hydrogen production rate of 1T-MoS 2 /TiO 2 was 5 and 8 times higher than those of bare TiO 2 and 1T-MoS 2 /TiO 2 [125].In another research, 1T-MoS 2 coated onto TiO 2 (001) composite (MST) was synthesized.DFT calculations suggested a closer distance between the interface electrons and MoS 2 surface than that of TiO 2 [131] (Figure 19).Therefore the photo-induced electrons can easily transfer to the conducting channel of MoS 2 .Furthermore, the introduction of 1T-MoS 2 prolonged the carrier lifetime remarkedly.All the factors led to an enhanced photocatalytic activity.
To further inhibit the recombination of electron-hole pairs, cocatalyst such as graphene is applied to MoS 2 /TiO 2 system [115,135,136].Xiang et al. employed TiO 2 /MoS 2 /graphene composite as photocatalyst [135].In this system, photo-inducted electrons transfer from VB to CB of TiO 2 .Then the electrons are further injected into the graphene sheets or MoS 2 nanoparticles.What is more, graphene sheets can be seen as electrons transport 'highway' through which electrons move from VB of TiO 2 to MoS 2 (Figure 20).The cocatalyst of MoS 2 and graphene enhances the interfacial charge transfer rate, inhibits the recombination of electron-hole pairs and offers a host of active site for adsorption and reaction.Han et al. constructed 3D MoS 2 /P25/graphene-aerogel networks.In addition to the above-mentioned advantages, 3D graphene porous architecture has a highly porous ultrafine nanoassembly network structure, excellent electric conductivity, and the maximization of accessible sites [115].Recently, a 3D double-heterostructured photocatalyst was constructed by connecting a TiO 2 -MoS 2 core-shell nanosheets (NSs) on a graphite fiber (GF@MoS 2 -TiO 2 ) [136].Mechanism of photocatalytic decomposition of dyes under both visible light and UV light was discussed (Figure 21).Anatase TiO 2 has a wide band gap (2.96 eV), while the band gap of MoS 2 is 1.8 eV.Because of the moderate bandgap of MoS 2 , the electrons can be irradiated from VB to CB of MoS 2 and then inject into CB of TiO 2 or transfer to graphene through intimate double-heterojunction contact under visible light.Graphene acts as electrons accepter under both circumstance, leading to a high rate of charge separation and thus depress the charge recombination.The contact interfaces and synergy among graphene, TiO 2 and MoS 2 play an important role in the superior photocatalytic activities.
While the transfer of electrons are paid special attention, the role of capturing the holes are often ignored.To solve this problem, a TiO 2 /WO 3 @MoS 2 (TWM) hybrid Z-scheme photocatalytic system was structured.TiO 2 and WO 3 have the appropriate energy level matching to form the Z-scheme, while the position of VB in WO 3 is lower than the VB of TiO 2 , and the CB of WO 3 is between the CB and VB of TiO 2 [137].Under UV light irradiation, the VB electrons of all three parts are excited to corresponding CB level.The excited electrons on CB of TiO 2 then transfer to CB of MoS 2 for H 2 evolution, meanwhile the excited electrons on CB of WO 3 were inject to the VB of TiO 2 (Figure 22).This procedure suppressed the recombination of photoinduced electrons and holes in TiO 2 , and therefore the photogenerated electrons and holes can be efficiently separated, which further leads to effective photocatalytic activity [137].

Conclusions
The coupling between TiO 2 and 2D material has proven to be an efficient approach to enhanced photocatalytic activity.Different methods vary the structures and surface contact of the hybrid and thus can modify the carrier separation process.The synergistic effects show that 2D material plays a vital role in photocatalysis when composited with TiO 2 .First, 2D material can act as electrons accepter or bridge to conduct photoinduced electrons, and therefore represses the recombination of carriers efficiently.Second, the gigantic surface of 2D material provides substantial active sites for substrate capture and reaction, not to mention rapid electrons transfer rate.Third, the 2D material can be decorated to obtain expected properties, for example, non-metal doping to adjust the energy level, specific crystal structure to short the pathway for interfacial charge transfer, and defects or introduced functional group for substrate trapping.What's more, the interfacial heterojunction can adjust energy level to broaden light response range and improve solar utilization.To further enhance the separation efficiency of electron-hole pairs, other photocatalysts are introduced to construct co-catalyst systems among which Z-scheme system can raise the hole trapping rate to some extent, and thus offers a new point to improve the separation of carriers.All factors mentioned above highlight the critical role of 2D material in photocatalyst and the 2D material/TiO 2 hybrid is worth to get further insight for a wider range of applications.

Figure 9 .
Figure 9. Schematic illustration for the possible mechanism of the visible light-driven photocatalytic degradation for the GD-NTNS composites.Reprinted with permission from [86].Copyright 2018, Springer.

Figure 9 .
Figure 9. Schematic illustration for the possible mechanism of the visible light-driven photocatalytic degradation for the GD-NTNS composites.Reprinted with permission from [86].Copyright 2018, Springer.